Hi jrdtech,
You can see the guidelines I use for memory foam mattresses in post #10 here.
A mattress would need to “pass” all 4 guidelines for me to consider it. The only one of the ones you mentioned that does is Restava (which are on the list of some of the better online memory foam options in post #12 here. I would keep in mind though that the two lowest priced mattresses (the Monaco and the Regina) don’t have a return policy, are non refundable.
Once you know the layering of each mattress and the foam densities then it’s much easier to make meaningful comparisons between mattresses by comparing the materials.
I would also make sure you are dealing with someone who has the experience and knowledge to help you choose a mattress that is suitable for your body type, sleeping positions, and preferences. People who are more interested in selling you whatever they can instead of what you really need are not worth dealing with. No matter how little you pay … if what you buy isn’t suitable for you then you would have wasted your money buying a “cheap” mattress that ends up being very expensive. If you buy low quality materials that soften and degrade quickly then that’s another way that “cheap” mattresses can be very costly both in terms of your quality of sleep and in terms of your wallet.
Phoenix